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Washington (CNN)The advertised purpose of President Barack Obama's pre-vacation news conference Thursday was to highlight gains in the war against ISIS in Iraq and Syria with the Pentagon as a backdrop.

But for all his talk of terrorism, Russia and other geopolitical challenges, the underlying argument he wanted to make was clear: it's a serious business being commander-in-chief and the Republican who wants his job isn't fit for the Oval Office.

Obama ridiculed Donald Trump's recent suggestion that the election system could be rigged, called on the candidate to act like a president since he's soon to be briefed on confidential information and implied that he didn't believe the billionaire businessman could be trusted with America's nuclear codes.

"Just listen to what Mr. Trump has to say and make your own judgment with respect to how confident you feel about his ability to manage things like our nuclear triad," he said in response to a question from CNN's Barbara Starr. "This is serious business."

Unserious, he suggested, was Trump's warning that November's election could be rigged against him: He mocked the assertion as "ridiculous."

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"Of course the election won't be rigged. What does that mean?" Obama said, struggling to disguise his contempt. "If Mr. Trump is suggesting that there is a conspiracy theory that is propagated across the country, including in places like Texas where typically it is not Democrats who are in charge of voting booths, that's ridiculous. That doesn't make any sense."

Obama, in his last planned public appearance before his annual vacation in Martha's Vineyard, said he had never heard of anyone complaining that they had been cheated before the score had been tallied.

"My suggestion would be, you know, go out there and try to win the election."

"If Mr. Trump is up 10 or 15 points on Election Day and he ends up losing, then, you know, maybe he can raise some questions," Obama said. "That doesn't seem to be the case at the moment."

Obama speaks out

It was the latest in a string of recent presidential rebukes to the GOP nominee. Obama hit out at Trump just two days after branding him unfit to be commander in chief from the podium in the White House's East Room. His intervention underscored the unusually prominent role the current occupant of the Oval Office is playing in the election to decide his successor as he tries to ensure that Democrat Hillary Clinton wins in November.

Obama did say, however, that should Trump beat Clinton, he would fulfill his duty to help the incoming president despite his criticisms of the billionaire's fitness for the Oval Office.

"If somebody wins the election and they are president, then my responsibility is to peacefully transfer power to that individual. And do everything I can to help them succeed," he said.

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But, he said, "We're going to go by the law which is that -- tradition and the law -- that if someone is the Republican nominee for president, they need to get security briefings so if they were to win, they are not starting from scratch in terms of being prepared for this office."

According to legal experts, however, there is no law requiring security briefings of presidential candidates, just a decades-long practice of doing so.

"They have been told: These are classified briefings. If they want to be president, they have got to start acting like (a) president. That means being able to receive these briefings and not spreading them around," he said.

Trump and Clinton are expected to soon start getting classified intelligence briefings in the run-up to the election. Some Trump critics have contended that the billionaire's unrestrained tongue could put US secrets in danger.

Republicans, meanwhile, have argued that Clinton should be barred from the briefings, saying she put classified information at risk through her use of a private email server for official business while secretary of state.

Though the briefings don't go into sources or methods of collecting intelligence, as they are considered extremely sensitive, they will offer a broad look at the threats and challenges facing US national security.

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ISIS is top challenge

Obama pointed to many such challenges in Thursday's Pentagon news conference, particularly the fight against ISIS, also known as ISIL.

"In fact, the decline of ISIL in Syria and Iraq appears to be causing it to shift to tactics that we've seen before, an even greater emphasis on encouraging high-profile terrorist attacks, including in the United States," Obama said.

He warned that there remained a "serious" threat of an ISIS attack on US soil, that would not be spectacular, like al Qaeda's assault on 9/11 but could be a smaller scale operations using "small arms, or assault rifles," or a truck, as happened recently in Nice, France.

The President, who has been accused by critics of not doing enough to stop the splintering of Syria amid a vicious civil war, indicated that he was always searching for a better policy.

"A big chunk of my gray hair comes out of my Syria meetings," Obama said, who held a meeting of his National Security Council at the sprawling military headquarters outside Washington before addressing the media.

"There's not a meeting that I don't end by saying, "Is there something else we can be doing that we haven't thought of? Is there a plan F, G, H, that we haven't thought of?'"

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Obama warned that if Russia did not help deliver Syrian President Bashar al-Assad for a ceasefire deal in his civil-war wracked country, it would expose itself as an irresponsible actor on the world stage.

"I'm not confident that we can trust the Russians and Vladimir Putin, which is why we have to test whether or not we can get an actual cessation of hostilities that includes an end to the kind of aerial bombing and civilian death and destruction that we've seen carried out by the Assad regime," he said.

Obama additionally offered a survey of diverse military operations around the world, including the fight to support the Afghan government in its civil war with the Taliban and a growing ISIS presence. He also mentioned the latest expansion of anti-ISIS operations in Libya as government forces there try to take back the city of Sirte from the radical Islamic group.

No ransom

Obama also forcibly denied that his administration had effectively paid a ransom to Iran after delivering $400 million of a $1.7 billion payment to settle a decades-old dispute over an arms deal on the same day that four US prisoners were freed in January.

"We do not pay ransom. We didn't here and we won't in the future," Obama said, adding that he had been open about the deal at the time and that recent reports about it simply offered new details about the transfer, carried out with a plane ferrying palettes of foreign currency.

As he celebrated his 55th birthday on Thursday, Obama can bask in the benefit of rising poll numbers. A new CNN/ORC poll finds that they stand at their highest level since just before his second inauguration in 2013.